Junior Dos Santos, left, and Shane Carwin face off after...

Junior Dos Santos, left, and Shane Carwin face off after a UFC 131 news conference in Vancouver, British Columbia. (June 9, 2011) Credit: AP

VANCOUVER, British Columbia -- Shane Carwin was supposed to ease back into the UFC. The heavyweight contender had been out for nearly a year recovering from back surgery for nerve damage in his neck, and he scheduled a comeback fight against Jon Olav Einemo, a Norwegian newcomer.

Brock Lesnar's latest bout with intestinal illness changed everything for Carwin, the 36-year-old mechanical engineer from rural Colorado with two of mixed martial arts' most devastating fists.

Instead of easing into the UFC shallows, he's jumping right back into the deep end against Brazil's Junior Dos Santos (12-1), widely considered the world's No. 2 heavyweight behind UFC champion Cain Velasquez.

When Lesnar dropped out of his fight with Dos Santos, Carwin (12-1) stepped in for the main event of UFC 131 Saturday in Vancouver's Rogers Arena, with a fall title shot going to the winner.

"You get very few opportunities like this in life," Carwin said. "I get to go in there and make the most of it and do something that I love to do. When I got that call, it was an absolute yes. Not only do I get to fight one of the best guys in the world, but it's with the title shot on the line."

Carwin was one punch away from winning that title last summer when he battered Lesnar for nearly every minute of the first round of their bout.

Lesnar spent the round curled in a corner of the cage, blocking punches and hoping Carwin would tire himself out. Carwin did just that, and Lesnar finished him in the second round.

While Carwin dealt with neck pain that eventually led to surgery in the ensuing months, he also realized he had to change his conditioning to compete with Velasquez or Dos Santos, two prodigious natural athletes. His shape improved immediately, and he didn't expect to need to cut weight before Friday's weigh-in.

"My diet is mostly organic now, a lot of fruits and vegetables," Carwin said, praising his nutrition consultant, Josh Ford. "We get some chicken and fish in for protein, but he completely changed my thoughts on nutrition and everything we do is for recovery. ... Every time you come out of a fight, you always try to make yourself a better fighter. There were a lot of things that I learned in that space."

MMA-mad Canada is hosting its latest UFC show just one day after the Vancouver Canucks play a pivotal Game 5 against the Boston Bruins in the Stanley Cup finals in the same downtown arena.

Veteran featherweight Kenny Florian meets Brazil's Diego Nunes earlier on the card, while lightweight contender Donald "Cowboy" Cerrone takes on Vagner Rocha before a bout between middleweights Demian Maia and Mark Munoz.

Dos Santos was looking forward to a bout with Lesnar after appearing together on the most recent season of "The Ultimate Fighter," the UFC's reality show. The UFC also was disappointed when its biggest pay-per-view star had another setback with diverticulitis last month, eventually undergoing surgery to remove a portion of his colon.

Dos Santos also hasn't fought in 10 months since winning the first decision of his career over Roy Nelson in Oakland, Calif., last August. He's an outstanding striker whose sheer size would be a problem for most heavyweights -- although Carwin matches up well.

"I believe so much in my boxing," Dos Santos said. "My boxing brought me here until now. I feel very comfortable fighting on my feet, so I will try to keep this fight standing, but I'm ready to fight whatever the fight needs, on the ground or on my feet. It doesn't matter where. I'm ready to fight."

Just how good are these two heavyweights on their feet? Dos Santos' last win was his first career fight that didn't end in a knockout or submission, while Carwin had never even gone beyond the first round of a fight until Lesnar beat him last year.

"Let's face it, both Dos Santos and I got to where we're at by knocking people out on our feet," said Carwin, who dropped out of a prospective bout with Nelson on New Year's Day due to his surgery. "I think we're both explosive fighters. I think the fans, they're going to have a real treat when it gets to that main event, knowing both guys have knockout power in their hands."

More MMA